Metallaboratrane compounds are a class of molecules with
a cage-like structure that feature metal-to-ligand MB s–dative bonds, as illustrated below.

While dative bonding is a common
feature of transition metals, the metal is normally the electron pair acceptor, rather than the donor.Of
particular interest, therefore, is the nature of the bonding in {[B(mimR)3]M}
complexes and the electronic impact that the borane ligand exerts on a metal
center.In this regard, two
important factors pertaining to the coordination of any ligand to a metal
center are the effects that it has on (i) the electron count and (ii)
the dn configuration, both of which play an important role in
evaluating the stability and reactivity of a molecule.

The essential feature of a general
M–B bonding interaction is illustrated by the simplified molecular
orbital diagram shown below.

Thus, the interaction between a
filled metal based orbital and the empty orbital on boron results in a filled
M–B bonding orbital and an unoccupied M–B antibonding orbital.Since the M–B bonding orbital is
occupied by a pair of electrons that were originally on the metal, a metal
center that originally possessed a dn configuration becomes dn–2
upon coordination to boron.The
transition from a dn to dn–2 configuration upon
coordination of a BR3 fragment bears a close analogy to the change
resulting from the interaction of a metal center with another Lewis acid,
namely H+.Thus, it is
widely recognized that protonation of a dn metal center results in
the formation of a metal–hydride in which the metal center has a dn–2
configuration.

The reactivity of the MB bond in
metallaboratranes has received very little attention. Therefore, we have started to
investigate the chemistry of metallaboratranes, with the intention of
uncovering new reactivity that is associated with cleavage of the MB bond.

We recently described that the FeB bond of
the ferraboratrane [k4-B(mimBut)3]Fe(CO)2
could be cleaved by a variety of reagents to give B–functionalized tris(mercaptoimidazolyl)borate complexes of the type [YTmBut]FeZ.For example, [k4-B(mimBut)3]Fe(CO)2
reacts with (i) CHX3 (X = Cl, Br) to give [XTmBut]FeX, and (ii) I2 in CHCl3
to give [ClTmBut]FeI.

In a similar manner, the NiB bond of
the nickel boratrane compounds [k4-B(mimBut)3]NiX (X = Cl, OAc, SCN,
N3) may also be cleaved by suitable reagents to afford B–functionalized derivatives, [YTmBut]NiZ.

For example, [k4-B(mimBut)3]NiCl reacts with CHCl3,
CHBr3, and I2 to give [ClTmBut]NiCl, [ClTmBut]NiBr,
and [ClTmBut]NiI,
respectively.Likewise, the azide
and isothiocyanate complexes [k4-B(mimBut)3]NiN3 and [k4-B(mimBut)3]NiNCS react with I2
to give [N3TmBut]NiI
and [SCNTmBut]NiI.In each of these examples, the boron is
functionalized by the ligand originally attached to nickel.It is, therefore, significant that [k4-B(mimBut)3]NiCl reacts with XeF2
to yield [FTmBut]NiCl in
which the boron is functionalized by the reagent and the chloride ligand
remains attached to nickel.